Until recently, the measurement of ocean currents and other liquid velocity measurements have been performed using mechanical meters equipped with Savonious rotors and vane followers. These methods of measuring currents have significant problems, including poor reliability due to the direct exposure of mechanical moving parts to the marine environment resulting in corrosion and fouling by extraneous matter, and the non-ideal hydrodynamic properties of these mechanisms which result in inaccurate measurements. Various attempts have been made in the past to design better instruments. These designs have utilized acoustic, electromagnetic, and various other electronic sensing techniques which have eliminated some of the problems associated with mechanical current measuring devices. However, these recent systems have significant problems with their use including zero drift, high power consumption, inoperability in clear water (for acoustic backscatter current meters), and low sensitivity.
Acoustic current meters have been implemented in a number of different ways, including: (1) direct measurement of the propagation time of a pulse emitted by a first transducer and received by a second transducer; (2) dual "sing-around" sound velocimeters with straight line sound paths in opposite directions, the difference in "sing-around" frequency being a linear function of the current; (3) continuous wave systems using two widely different high frequency carriers (e.g., 1.1 and 1.6 MHz) but modulated with an identical signal of lower frequency (e.g. 20 kHz) where the phase difference of the modulating signal on the received carriers is a linear function of current velocity; and (4) continuous wave bursts using a single frequency on a single pair of transducers, the burst interval being approximately equal to the acoustic travel time between the two transducers.